Solar cells provide a means to produce electric power with minimal environmental impact because it is a renewable technology. In order to become a commercial success the solar cells need to be efficient, to have low cost, to be durable, and not add other environmental problems.
Today's dominant solar cell technology is based on crystalline silicon. It fulfils many of the requirements mentioned above but can not be produced at such low cost that electricity generation in large scale is cost effective. It also requires relatively large amount of energy in the production, which is an environmental disadvantage.
Solar cells based on thin film technologies have been developed. They offer a potential of substantial cost reductions but have, in general, lower conversion efficiencies and less good durability. A very promising thin film solar cell technology is based on the semiconductor Cu(In,Ga)Se2, abbreviated CIGS, which has demonstrated high efficiencies (16.6% in small prototype modules [1]) and durability in operation. It remains to demonstrate low cost in real production.
CIGS-solar cells are thin-film solar-cells with a CIGS-layer serving as absorber of sunlight. Electron-hole-pairs are generated therein.
A typical CIGS-solar cell is shown in FIG. 1 and comprises a glass substrate 1 with a thickness of 2-3 mm, an Mo-back contact 2 with a thickness of 0.5-1 μm, a CIGS-layer 3 of 1.5-2 μm, a CdS buffer layer 4 with a thickness of 50 nm and a ZnO window layer 5 of 0.5-1 μm. An optional second buffer layer 6 may be present between the CdS buffer layer and the window layer and has a thickness of 50 nm.
The CIGS-layer is a p-conductive Cu(In,Ga)(Se,S)2 compound. The CdS buffer layer serves as protection of the CIGS-layer. The window layer is a n-type conductive doped zinc oxide layer. With the CIGS-layer it forms a pn-junction and serves as a transparent front contact. The optional second buffer layer comprises non-doped ZnO. Presently its role is not fully understood. Statistically seen, solar cells with this second buffer layer exhibit better properties compared to cells with a single ZnO-layer.
The commonly used way of fabricating CIGS solar cell modules include formation of a pn-junction and front contact according to the following: (1) a buffer layer (typically 50 nm of CdS) is deposited by chemical bath deposition (CBD), (2) a high resistivity thin layer of ZnO is deposited on top of the CdS layer by sputtering, (3) the layered structure is patterned by mechanical scribing to open contacts for the serial interconnects, (4) a front contact of a transparent conductive oxide (TCO) is deposited, and (5) an additional patterning step of mechanical patterning as part of the interconnect structure.
EP-A2-0 838 863 discloses a thin-film solar cell fabricated on a glass substrate. The solar cell comprises a metallic back electrode, a light-absorbing layer having on its surface a Cu(InGa)(Se)2 layer (CIGS layer), an interfacial layer referred to as a buffer layer, a window layer and an upper electrode. The back electrode is a p-type semiconductor and the window layer is an n-type semiconductor. Between the back and upper electrodes an open circuit voltage in the order of 0.2-0.8 V is obtained when the solar cell is hit by light. Electrical current will be generated in the p-n junction between the p- and n-layers.
The buffer layer comprises a group II-VI compound semiconductor containing hydroxyl groups. An exemplary compound is Zn(O,S,OH)2.
DE 44 40 878 C2 discloses a thin-film solar cell comprising a glass substrate, a back electrode, a light absorbing layer, a front buffer layer and a window layer. The front buffer layer comprises a mixture of In(OH)3 and In2S3 and is applied in a wet process or a chemical vapour deposition process (CVD) with organo metallic compounds or with atomic layer epitaxy deposition process (ALE). The novel concept here is that the buffer layer does not contain cadmium.
To fabricate the solar cell with chemical wet process steps mixed with sputtering makes the solar cell less attractive for large scale production. To fabricate the solar cell wet processes are mixed with the co-vapour deposition process which renders the structure less attractive for large scale production.
An example of thin-film solar cells using a first CdZnS buffer layer between a CIGS-layer and a window layer is disclosed in U.S. Pat. No. 5,078,804. A second ZnO buffer layer on top of the first buffer layer and in contact with the window layer is also provided. The first CdZnS buffer layer comprises two layers, a high Zn content CdZnS layer on top of a low Zn content CdZnS layer, both of which are applied using an aqueous solution method. The second ZnO buffer layer also comprises two layers, a low resistivity ZnO layer on top of a high conductivity ZnO layer both of which are applied using sputtering in an argon or oxygen/argon atmosphere.
To manufacture the solar cell with chemical wet process steps mixed with sputtering makes the solar cell less attractive for large scale production. The use of toxic cadmium makes the cell less attractive for environmental reasons. Also handling and disposal of the hazardous wastes are costly.
U.S. Pat. No. 5,948,176 discloses a solar cell structure comprising a first active CIGS layer on top of a metal back contact. On top of the CIGS layer a buffer layer of n-type conductive doped ZnO layer is deposited by a wet process using a zinc chloride solution as doping source. A second active layer comprising ZnO is deposited on the buffer layer. The second active layer comprises a first high resistive ZnO layer and a second low resistive ZnO layer on top of which a grid of front face electrodes is sputtered.
Another example of a cadmium containing solar cell is shown in U.S. Pat. No. 4,611,091.